The present invention relates to static convertor equipment for transmitting energy between an AC network and a DC network, and, more particularly, to such equipment including a convertor with forced commutation that is connected to an AC network through an inductance element, and to a DC network having a smoothing capacitor and a filter connected in parallel with the capacitor, the filter having an inductor that is connected in series with a filter capacitor.
Static convertor apparatus for transmitting energy between an AC and a DC network is known to the art, for example such apparatus is disclosed in the published German patent application No. 2,217,023. For such apparatus, the filter circuits connected on the DC side of the convertor may be assumed to maintain the direct voltage substantially constant. The alternating voltage generated by the convertor may then be controlled, for example by pulse width modulation of the convertor, so that the voltage becomes approximately sinusoidal and has the same frequency as the frequency of the AC network. By controlling the amplitude and the phase position of the alternating voltage of the convertor, the amplitude and the phase position of the alternating current flowing between the AC network and the convertor may be controlled and, thus, the magnitude and direction of the active and reactive power flowing between the AC network and the convertor equipment may be individually and arbitrarily controlled. In this way, complete control is obtained of the power that is transmitted between the AC and the DC networks, regardless of the direction of the power flux. At the same time the equipment may be controlled so that its reactive power consumption is maintained at a desired value, for example zero.
The inductance element that is connected between the convertor and the AC network and that takes up instantaneous voltage differences between the DC and AC voltages of the convertor may consist of a separate inductor or of the inductance of a transformer that is used for connecting the convertor to the AC network.
Equipment of the kind described above may, for example, be used to supply an AC motor with a variable frequency from an AC network with a constant frequency. The DC network then consists of an inverter connected to the DC side of the convertor.
A smoothing capacitor connected on the DC side of the convertor limits variations in the direct voltage. The direct current of the convertor will contain an AC component with a fundamental tone that has a frequency equal to the frequency of the AC network, multiplied by the pulse number of the convertor. If the convertor is a single-phase convertor (low pulse number) and the frequency of the AC network is low, the fundamental tone of the AC direct current component will have an amplitude that is of the same order of magnitude as the mean value of the direct current and it will have a low frequency.
In order to keep the variations in the direct voltage within reasonable limits an unrealistically high capacitance for the smoothing capacitor is often required or, at least, the required high capacitance seriously limits the maximum convertor power that may be installed, for reasons of space and weight.
From the above-mentioned German application No. 2,217,023 it is known to tune an LC filter to twice the power frequency and to connect the filter in parallel with a smoothing capacitor. In this way, the required capacitance of the smoothing capacitor is reduced, since the filter can be operated to completely eliminate the voltage variations that are due to the fundamental tone component of the AC direct current component of the convertor. The smoothing capacitor then only needs to damp the current harmonics of a higher order. However, for such prior art circuits, the necessary smoothing components (the smoothing capacitor and the filter) become undesirably large and heavy, particularly in situations where the equipment must be built into a vehicle. Thus, the rated power of the filter components only, at the frequency of the fundamental tone, will be of the same order of magnitude as the maximum active power for which the convertor is dimensioned.
Furthermore, for prior art filters there is the added risk that variations in operating temperature may change the tuning of the filter so that the filtering becomes less efficient. Accordingly, it is a primary object of the invention to provide a static convertor apparatus that transmits energy between an AC and a DC network and that employs filter components of reduced size.
A further object of the invention is to provide such an apparatus that will not be affected in operation by variations in the impedance values of filter components due to such factors as temperature or aging.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.